1. Field of the Invention
Embodiments of the present invention are directed to a probe card assembly and more particularly to a method and apparatus for providing a probe card assembly with a precisely fixed probe substrate position relative to the reference plane of the probe card.
2. Description of Related Art
A modem probe card assembly used to test wafers of semiconductor chips generally consists of a Printed Circuit Board (PCB) (also referred to as a printed wiring board or probe card wiring board), a probe contractor substrate having probes for contacting the wafer (sometimes referred as a probe head), and an interposer connecting the PCB to the probe contractor substrate.
Probes (also referred to as probe contactors) are generally compliant mechanisms including at least one spring which have some limited range of compliance in a vertical direction (the “z” direction). Some probes have minimal or no compliance. When in use, a wafer under test is urged upward to contact the tips of the probes. In practice, there is some manufacturing process-related z error (non-planarity of the probe tips) caused by film stresses, etch control, assembly control, etc. as well as systemic z errors caused by a warping or curving in the surface of the probe contractor substrate. If the probe contractor substrate is curved or warped, so will be the imaginary surface that goes through the tips (assuming that the probes are of uniform height). Thus some probe tips will contact the wafer first (called the first touch z height) and some probe tips will contact the wafer last (last touch z height). Because probes generally have a limited range of compliance (as small as 50 μm or less for many microfabricated technologies), it is desirable to minimize both the process-related and systemic errors in tip z height. Some errors are most directly related to the fabrication of the probes on the probe contractor substrate rather than the probe card assembly design. However, some errors are usually directly related to the probe card assembly and the way the PCB is mounted with the probe contractor substrate or substrates. The minimization of these latter errors is the subject of the present invention.
In older probe card applications, a prober has a surface which has been planarized to that of the chuck that carries the wafer under test. The probe card PCB is generally mounted to this planarized surface of the prober. Thus, all such probe card assemblies require well controlled parallelism between the plane of the probe tips (the best-fit plane that minimizes the overall root-mean-square z error between the probe tips and the plane) and the plane of the PCB (the PCB can be thought of as the “reference plane.” If the probe tips are co-planar with the PCB, then they are also co-planar with the chuck, and thus with the wafer under test). Such a design will lead to a more uniform contact of the probes to the wafer under test (less of a distance between first touch z distance and last touch z distance). In newer probe cards, the probe tips are referenced to mounting points on the probe card which are typically kinematic mounts of some type (used here to describe a mount that provides accurate and repeatable mechanical docking of the probe card into the test equipment and provides constraint in at least the three degrees of freedom necessary to achieve parallelism to the plane of the wafer chuck). In either embodiment, it is necessary to align the tips of the probe contactors such that they are parallel to a reference plane which is itself parallel to the plane of the wafer chuck.
There are two common ways that a probe contractor substrate may be mounted to the probe card assembly (which includes the PCB, an associated stiffener ring and/or other mechanical elements) in a planar manner: Fixed Probe Card Assemblies (FPCAs) and Adjustable Probe Card Assemblies (APCAs). FPCAs provide for design simplicity (no moving or adjustable parts) and relatively low cost. However, the machining tolerances required for parallelism, particularly in the case of large area probe cards, can be difficult to achieve. Hence, in practice, shims are often used to provide some degree of adjustability during assembly. Shimming; though a practical alternative, is difficult to perform accurately in a manufacturing environment to the tolerances required (on the order of microns).
FPCAs include typical “Buckling Beam” assemblies, such as that shown FIG. 1A and described in U.S. Pat. No. 3,806,801 entitled “Probe Contractor Having Buckling Beam Probes.” Buckling Beam assemblies have a vertical buckling beam probe head, a PCB, and an interposer situated between the probe head and the PCB. In this case the interposer comprises an array of solder balls which electrically connect the substrate to the PCB (terminal to terminal) but other examples are well known in the art where the interposer connects the substrate to the PCB by means of spring-pins. In another version of the Buckling Beam probe card, there is no interposer and the buckling beams connect directly to the terminals of the PCB.
In either type of Buckling Beam assembly the probe head is made to be parallel to the PCB surface by first machining the head so that the surfaces are parallel, and second by shimming between the head and the PCB. It is also common practice in the art to mount the probe card and lap the probe tips parallel to the mount, though this technique introduces unwanted damage to the probe tips and is not practical for coated probes (probes with a thin coating of material that is different from the base spring material).
APCAs are well known in the art and range from providing for small groups of adjustable pins to entire probe-bearing substrates panels or assemblies which are adjustable in place relative to the card's reference plane. The unifying characteristic is that a mechanism is provided for moving groups of probes relative to the probe card reference plane while maintaining electrical contact between them. The advantage of adjustability is that parallelism can be readily achieved, even in the field between uses or during use. However adjustability also has a number of significant disadvantages including drift of the adjustment over time and thermal cycling, cost of the relatively complex precision mechanical assemblies required and difficulty of assembly. Furthermore, the adjustment mechanism can take significant space and limit the density of adjacent blocks of probes.
U.S. Pat. No. 5,974,662 entitled “Method of Planarizing Tips of Probe Elements of a Probe Card Assembly,” as shown in FIG. 1B, describes such an APCA and discloses a method of making a probe card with an adjustment of the probe tips relative to the probe card assembly's reference plane. The assembly incorporates a space transformer substrate which is mounted to the probe card in such a way that the orientation can be adjusted. A vertical spring interposer is used to electrically interconnect the probe contractor substrate to the PCB and differential screws bearing on the substrate provide the adjustability. This particular design is particularly expensive, difficult to assemble, and complex. In addition, the large number of mechanical components required to achieve adjustability make the design inherently thermally unstable.
In some cases, it is desirable to have multiple “tiles” of probe card substrates (each with a plurality of probes) attached to the PCB. The assembly of probe card substrates of onto larger assemblies may be accomplished in a variety of ways. Most of these assemblies fall into one of two categories: Fixed Assemblies where the substrates are individually fixed to a carrier without further alignment; and Aligned Assemblies where the substrates are mounted to the card on an adjustable leveling mechanism. Fixed assemblies rely on the tolerances of the various elements and the tooling used to set the overall tolerance of contact points on the various substrates relative to one another. For reference, the desired tolerance in all three orthogonal directions is on the order of ±5 μm, which number is very difficult to achieve through a fixed assembly. U.S. Patent Publication No. 20040163252, assigned to Form Factor International, is an example of a fixed assembly, as shown in FIG. 2A.
Adjustable assemblies typically require some form of macroscopic adjustable mount with a full six degrees of freedom in order to align substrates relative to one another. The trouble with this method is that the mount is relatively large (so that it does not fit in the conventional envelope provided for probe cards) cumbersome, expensive and unstable (i.e. drifts in position as a function of time, particularly when exposed to thermal excursions). U.S. Pat. No. 5,091,694 entitled “Quartz Probe Apparatus,” is an example of an adjustable assembly, as shown in FIG. 2B.
Thus, what is needed is an improved probe card assembly and less expensive, yet stable method of planarizing the probe head(s) to the PCB or other reference plane for such a probe card assembly.